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PostPosted: 27 May 2019, 18:19 
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If you calculate the cut off frequency whenver the signal has to pass through a capacitor then you can indentify what went wrong

I generally find anything with cut off of 9hz or lower to be good
anything 0.5hz or lower tends to be too small and detoriates sound quality and may cause instablity

You probally wouln't have gone wrong withe the valve pre ampdesign, but I'm guessing the pre amp to op amp capacitor is probaly size incrrrectly.
Those feedback resitors on op amp has to be 10s of ks to get ou 9hz cut off.
You can drop the gain and reduce f cut by introducing a serires resistor with the capacitor, or you can increase op amp voltage gain to increase input impedance

When you make the op amp feedback resistors too big you risk oscillation otherwise is just cut off earlier for high frequency

https://www.electronicproducts.com/RC_F ... lator.aspx
Though I know the formulas I still like to use this one online, is easier and faster


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PostPosted: 27 May 2019, 18:33 
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ILoveHiFi wrote:
If you calculate the cut off frequency whenver the signal has to pass through a capacitor then you can indentify what went wrong

I generally find anything with cut off of 9hz or lower to be good
anything 0.5hz or lower tends to be too small and detoriates sound quality and may cause instablity

You probally wouln't have gone wrong withe the valve pre ampdesign, but I'm guessing the pre amp to op amp capacitor is probaly size incrrrectly.
Those feedback resitors on op amp has to be 10s of ks to get ou 9hz cut off.
You can drop the gain and reduce f cut by introducing a serires resistor with the capacitor, or you can increase op amp voltage gain to increase input impedance

When you make the op amp feedback resistors too big you risk oscillation otherwise is just cut off earlier for high frequency

https://www.electronicproducts.com/RC_F ... lator.aspx
Though I know the formulas I still like to use this one online, is easier and faster


In the image values didn’t display. The output cap from the tube section is 0.68uf. The input R to the chip is 3.9k The feed back resistor is 3.9K. I’d guess and say a gain of 2. What if I increase the input resistor to 10k? Thanks for the calculator.

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PostPosted: 27 May 2019, 19:01 
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Try changing the two 3.9k feedback resistors to 27k, it should work since 27k is not a exxessivly large value. This gives you cut of of lower than 8.7hz calculated when you account output imedance of the valve gain stage

If you don't have 27k, any value like 22k 18k 33k should also do the job
This jacks up the input impedance without changing the gain

A exccesivly large value would be something like 100k or larger
Check for oscillations with the high frequency filtering turned OFF on the scope,
1x mode for probes desired as sometimes 10x proper with the Resizstor divider circuit filters out hf oscilaltions


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PostPosted: 27 May 2019, 20:10 
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ILoveHiFi wrote:
Try changing the two 3.9k feedback resistors to 27k, it should work since 27k is not a exxessivly large value. This gives you cut of of lower than 8.7hz calculated when you account output imedance of the valve gain stage

If you don't have 27k, any value like 22k 18k 33k should also do the job
This jacks up the input impedance without changing the gain

A exccesivly large value would be something like 100k or larger
Check for oscillations with the high frequency filtering turned OFF on the scope,
1x mode for probes desired as sometimes 10x proper with the Resizstor divider circuit filters out hf oscilaltions

It easy to add the extra resistance for testing. I’ll try it.

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PostPosted: 27 May 2019, 22:18 
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https://allenlu2007.files.wordpress.com ... =329&h=190
If the increases ressitance to 27k makes it slower and brings up some over shoot on the square
You can try to reduce over shoot on square and round it off by adding a capacitor few pf 5-330pf range, I would start with 5-33pF


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PostPosted: 27 May 2019, 22:46 
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ILoveHiFi wrote:
https://allenlu2007.files.wordpress.com/2015/06/mlenewimage33.png?w=329&h=190
If the increases ressitance to 27k makes it slower and brings up some over shoot on the square
You can try to reduce over shoot on square and round it off by adding a capacitor few pf 5-330pf range, I would start with 5-33pF


Made the changes yet to listen.

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PostPosted: 29 May 2019, 22:08 
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I listened to the amp with the extra 22K input resistors and thought the bass better. Listening to test tones 40hz and lower was clear but not sure how this compared to the unmodded version.

On the CRO there was no difference in the lower frequency response. I even shorted out the extra 22k and there was now lower frequency difference. But I may have made a mistake with the upper frequency roll off. It is more like 100KhZ+.

Regardless the amp is extremely enjoyable and I have listened to nothing else now for days.

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PostPosted: 30 May 2019, 04:20 
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Probally theres less load in the output caps from valve pre amp to op amp, improved sound.
Also the pre amp less loaded up, 22*2khz input impedance vs 3.9*2kHZ

But allot of eqipment don't even go below 40hz anyways and the music is mostly arround 40hz or higher on the lowest, unless you like the rap music for hommmies
If you have 22k a upper roll of like 100khz is normall, because large impedance the op amp input capacitance cannot be charged up quickly


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PostPosted: 04 Jun 2019, 15:47 
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Very nice build ! Must be very enjoyable to listen to something as silent as it seems to be. I wish my 4S preamp was as silent as this !


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PostPosted: 04 Jun 2019, 19:43 
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Gab wrote:
Very nice build ! Must be very enjoyable to listen to something as silent as it seems to be. I wish my 4S preamp was as silent as this !


Try moving the earth wire. On one build I put the earth on the RCAs to stop the hum. In another I moved the earth to the pot and not the RCAs. By the earth I mean the negative of the HT.

If you have the mains earth connected to the HT negative try removing it if not try connecting it. And do that in combination with the above two connections. Generally don’t have two earth connections by that I mean don’t connect to the RCAs and the pot. Or if you haven’t try both. All this varies from one built to the next and there never seems to be a set way.

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